1. Field
The present invention generally relates to ATSC digital television (“DTV”) and mobile/handheld (“M/H”) broadcast systems, and more particularly to data frame and trellis encoder synchronization.
2. Related Art
A single-frequency network (“SFN”) is a collection of transmitters operating on the same frequency for carrying the same information to receivers in a given area. The transmitters emit identical signals, several of which may be received more or less simultaneously by individual receivers. One application of SFNs is for transmission of digitally encoded data for digital television (“DTV”), the system and related standards for which have been established by the Advanced Television Systems Committee (“ATSC”). Under the ATSC's DTV standard (or A/53 standard), it is possible to transmit large amounts of data including high definition pictures, high quality sound, multiple standard definition pictures, and other ancillary related or unrelated communications, which may be accessible by using, for example, a stationary receiver such as a computer or television set. Such stationary receivers are also interchangeably referred to as “fixed” receivers. The data broadcasted to stationary receivers are interchangeably referred to as “normal data,” “main service data” and “main stream data.”
Another application of SFNs is for transmission of digitally encoded data for mobile/handheld (“M/H”) devices, the system and related standards for which are currently being established as a candidate standard, the document for which is designated the ATSC A/153 candidate standard (also referred to as the “M/H DTV candidate standard”). Under the A/153 candidate standard, M/H broadcasting services share the same RF channel as the ATSC A/53 broadcast service and are provided using a portion of the ˜19.39 Mbps of the ATSC 8-level vestigial sideband (“8-VSB”) signal bandwidth.
FIG. 1 depicts the system core functions and enhancement tools that have been added to the A/53 standard 102 to form the A/153 candidate standard 112. Generally, the A/53 candidate standard is based on the VSB modulation as is the standard ATSC A/53 broadcast service, coupled with additional forward error correction mechanisms and known training sequences to aid mobile receivers. As shown in FIG. 1, the A/153 candidate standard requires, among other things, known data frame offsets 104 to map data packets with the A/153 standard's frame structure, and pre-coder and trellis encoder initialization 106 (pre-coder and trellis encoder are collectively referred to herein as a “trellis coder”), which provides in part the mechanism for creating known training sequences. Techniques for providing VSB data frame and trellis coder synchronization are described in U.S. Pat. No. 7,532,677 (the '677 patent), which is hereby incorporated herein by reference in its entirety.
The system achieves the robustness needed for mobile reception by adding extra training sequences 108 and several levels of forward error correction (“FEC”), one at the packet layer by a 2D Reed Solomon/CRC code, and another at the physical layer by a serial concatenated convolution code (“SCCC”) 110, which is formed together with the trellis coding of the 8-VSB exciter. A parallel concatenated code (“PCCC”) is also used for robust signalling to the mobile receiver.
FIG. 2 depicts a block diagram of an ATSC M/H transmission system including a pre-processor 204 (also referred to as an “M/H multiplexer”) and a post-processor 2221, 2222, . . . , 222i, . . . , 222n. Generally, the M/H system is a dual-stream system including an ATSC service multiplexer for existing digital television services or “main service” data and an M/H service multiplexer for one or more mobile and handheld services or “M/H service” data. An example of such a transmission system is described in U.S. Pat. No. 7,532,857, which is hereby incorporated herein by reference in its entirety.
As shown in FIG. 2, the main service and M/H service multiplexers feed into a head-end 202 (e.g., in a studio) where signal processing of main and M/H service data is performed. The processed data is then communicated over a studio-transmitter link (“STL”) 2061, 2062, . . . , 206i, . . . , 206n to a post-processor 2221, 2222, . . . , 222i, . . . , 222n at a remotely positioned transmitter 2241, 2242, . . . , 224i, . . . , 224n. As described in the A/153 candidate standard, in pre-processor 204, M/H service data are encoded with a Reed-Solomon/CRC (“RS/CRC”) coding in the M/H Frame encoder 210, a serial concatenated convolutional encoder in the block processor 212. Pre-processor 204 also generates signalling information such as status data such as the length, the periodicity and the sequence number of data units for each service, the time marker for transmission time of each data unit, and so on which are PCCC (Parallel concentrated code) encoded by a signalling encoder 213 and combined with MH encoded data in the group formatter 214. The encoded M/H payload data containing M/H training signals, additional control and status data are formatted into a MH Group by the group formatter 214 and formed into Mobile Handheld Encapsulated (MHE) transport stream (TS) data packets with a packet identifier (“PID”) (in a normal ATSC TS header) by the packet formatter 216 at the end of pre-processing.
Markers in TS data packets with main service data are modified by a packet timing and program clock reference (PCR) adjustment unit 220 which performs packet timing and PCR adjustment. A consecutively positioned packet multiplexer 218 multiplexes the normal TS data packets with MHE TS data packets to form the nominal 19,392658 bit/s data rate specified in the A/53 standard.
At each remote positioned transmitter 2241, 2242, . . . , 224i, . . . , 224n, in the post-processor 2221, 2222, . . . , 222i, . . . , 222n, the normal data packets are channel coded as specified in A/53 to maintain compatibility with normal ATSC receivers. This includes stages which provide data randomization by a data randomizer 2261, 2262, . . . , 226i, 226n, systematic/non-systematic RS encoding by encoder 2281, data interleaving by interleaver 2301, 2302, . . . , 230i, 230n, and trellis encoding by trellis encoder 2341, 2342, . . . , 234i, 234n. All stages in the post processor 2221, 2222, . . . , 222i, . . . , 222n other than data interleaver 230i have a dual mode (i.e. normal/MHE) capability which is selected per the type of packet (normal/MHE) being processed. Each post-processor 2221, 2222, . . . , 222i, . . . , 222n is followed by a synchronization multiplexer 2401, 2402, . . . , 240i, . . . , 240n for inserting synchronizing data (e.g., data field segments and data segment sync). Signalling is inserted in the data field sync to signal to receivers when MH mode is active or not, a pilot inserter 2421, 2422, . . . , 242i, . . . , 242n for inserting pilot symbols in the transport data stream, an optional pre-equalizer 2441, 2442, . . . , 244i, . . . , 244n, an 8-VSB modulator 2461, 2462, . . . , 246i, . . . , 246n, an RF-up-converter 2481, 2482, . . . , 248i, . . . , 248n for RF signal processing and a transmitter antenna 2501, 2502, . . . , 250i, . . . , 250n.
Non-Systematic RS parity bytes are calculated and placed in known positions within each MH Group by Non-Systematic RS encoding in encoder 2281, 2282, . . . , 228i, 228n, these positions enable, in part, the generation of six long MH training signals in each MH Group. Each of the six training sequences begins with 12 initialization bytes (one for each 1 of 12 trellis encoders) and is used by modified trellis encoder 234i, 2342, . . . , 234i, 234n to initialize all trellis states to a known zero value before the following known training data begins to enter the trellis encoder. This action will create known repeatable training symbols for MH receivers. The encoder 2281, 2282, . . . , 228i, 228n inserts the non-systematic parity bytes prior to trellis initialization in modified trellis encoder 2341, 2342, . . . , 234i, 234n. Some parity will become erroneous after the training initialization bytes are processed (i.e., values are changed) in modified trellis encoder according to the A/153 candidate standard. Accordingly, modified trellis encoder 2341, 2342, . . . , 234i, 234n supplies these changed initialization bytes to a non-systematic RS encoder 2361, 2362, . . . , 236i, 236n, which (non-systematically) re-calculates the RS parity of corresponding M/H packets using the changed data and original packet data from data interleaver 2301, 2302, . . . , 230i, 230n. The new RS parity bytes obtained by performing the non-systematic RS re-encoding process are supplied to RS parity replacer 2381, 2382, . . . , 238i, 238n, which selects the output of the data interleaver 2301, 2302, . . . , 230i, 230n or the output of non-systematic RS encoder 2361, 2362, . . . , 236i, 236n with the re-calculated RS parity.
FIG. 3 depicts the structure of an M/H Frame according to the A/153 candidate standard. As shown in FIG. 3, a data stream of consecutively transmitted M/H data frames includes 5 M/H Sub-Frames. Each sub-frame contains 156 TS data packets and each TS data packet is 188 bytes. Each set of 156 TS data packets is referred to as an M/H Slot which can contain a combination of M/H data packets and normal data packets, or only normal data packets. In normal data packets, only digital data for stationary (or fixed) receivers are transferred, whereas M/H data packets contain only data for M/H receivers.
More specifically, an M/H Slot may contain 118 data packets with data for M/H receivers (i.e., “M/H Group”) and 38 data packets of normal data for stationary receivers, i.e., “normal 38 packets”. Alternatively, an M/H Slot may contain 156 data packets of normal data only (i.e., “normal 156 packets” with data for stationary or “main stream” receivers). The mapping of the received ATSC M/H packets to positions in an 8-level vestigial sideband (“8-VSB”) data field is shown in FIG. 4. One purpose for this data mapping is to ensure MHE TS packets sent from pre-processor 204 are synchronized in and with post-processor 2221, 2222, . . . , 222i, 222n in the MH exciter. This mapping also enables MH receivers to tune and select the MH data wanted during reception by ensuring the MH data will be at known symbol positions in the physical layer VSB Frame. As shown in FIG. 4, the 38th data packet (#37) in an ATSC M/H Group received in the first data slot #0 of a first sub-frame #0 in the received ATSC M/H data frame is mapped to the first position for a data packet in an odd VSB data field.
The 38th data packet in the ATSC M/H Group received in Slot #2 is mapped to the first position for a data packet in an even VSB data field. The 38th data packet in the ATSC M/H Group received in Slot #1 is mapped to the 157th position for a data packet in an odd VSB data field. And, the 38th data packet in the ATSC M/H Group received in Slot #3 is mapped to the 157th position for a data packet in an even VSB data field.
According to the A/153 candidate standard, an ATSC M/H Group with a data structure corresponding to FIG. 4 is created in the pre-processor 204 (FIG. 2) in the ATSC M/H channel of the head-end 202 (FIG. 2). In total, 45 dummy bytes are placed in the 118 MHE packets of an ATSC M/H Group. These A/153 dummy bytes are used as padding bytes and serve no other useful purpose. Some of these dummy bytes will be used in the present invention to create a point to point signalling channel between pre-processor and post-processor. There are 13 dummy bytes in the first two (2) MHE packets including eight (8) dummy bytes in the 1st MHE packet and five (5) dummy bytes in the 2nd MHE packet. When transmitted between transmitters and receivers, all these dummy bytes typically have a fixed pre-selected value, e.g. 0xAF.
Since the pre-processor 204 and post-processor(s) 2221, 2222, . . . , 222i, 222n (FIG. 2) are remote from each other, they need to be synchronized to one another (i.e., a deterministic mapping with a known packet offset between the start of a VSB field and the MHE packets carrying enhanced data must be set). There also needs to be a way for the post processor 222i to distinguish MHE packets and normal data packets when the packets are received at the exciter. Currently, neither the A/53 nor A/153-candidate standards provide mechanisms for signalling a mode or for synchronization between the pre- and post processors, or for identifying the MHE packets in the exciter. In addition, there is a need to provide a mechanism to switch between modes in ATSC SFN, particularly ATSC SFN with M/H services and ATSC SFN without M/H services, and vice versa. U.S. patent application Ser. No. 12/468,938 (the “'938 application”), filed on May 20, 2009, and hereby incorporated herein by reference in its entirety, describes example mechanisms for performing such synchronization and identification for a single transmitter.
As described in the '938 application, to synchronize the pre-processor with the post-processor, each exciter uses signalling information determined in the pre-processing stage at the head-end. This signalling information is inserted by the packet multiplexer (FIG. 2, 218) in particular byte positions in the MH Group that are set aside for dummy bytes specified in the A/153 candidate standard.